1. Field of the Invention
The present invention relates to an optical recording medium and to a record blank suitable for recording information with a laser beam, particularly, a semiconductor laser beam. It also relates an information record allowing optical reproduction of information recorded by a laser beam.
2. Description of the Related Art
Generally, information can be recorded on an optical recording medium, such as a series of optical discs or optical cards, by forming optically detectable minute pits: (for example, of approximately 1 .mu.m) arranged in such a manner as to define a spiral, concentric or linear track on a recording layer formed on a substrate of the recording medium, thereby making it possible to store information at high-density.
As described, for example, in "Review and Analysis of Optical Recording Media" Optical Engineering, Vol. 15, No. 2, March-April 1976, pp99-108, a known optical recording method consists in applying a light beam, such as a laser beam, to the recording layer of an optical recording medium so as to generate deformation or pits on the recording layer. In other known methods, the application of such a light beam causes the generation of bubbles, changes in phase, discoloration or the like.
Various materials have been proposed for the recording layer of such an optical recording medium. Examples of such materials include inorganic materials, such as a metal film consisting of an aluminum deposit film, a bismuth film, a tellurium oxide film, or a chalcogenite-type non-crystalline glass film. Generally, such thin films are sensitive to light having a wavelength of approximately 350 to 800 nm and exhibit high reflectance to laser beams. One disadvantage of these films is that they provide a rather poor laser-beam-utilization factor. Further, such an inorganic material thin film is usually formed as a recording layer by sputtering. Such a film formation method, however, requires a production line equipped with a vacuum system, so that these inorganic material thin films have higher production costs.
In view of the above problems, optical recording mediums using organic coloring matter whose optical properties can be changed by the energy of light having a relatively long wavelength (for example, 780 nm or more) are being carefully studied. Optical recording mediums using such organic coloring matter are effective in that they allow the formation of pits by a semiconductor laser having an oscillation wavelength of around 780 nm or 830 nm. Moreover, organic coloring matter allows film formation by wet coating and can be easily treated, making it possible to easily mass-produce recording mediums on less expensive equipment.
The applicant of this invention discloses, in U.S. Pat. Nos. 4,944,981 and 5,079,127, polymethine dyes which permit recording by a semiconductor laser at a high S/N ratio and which provide an optical recording medium having good thermal stability.
In recent years, however, a decrease in the size of a record/reproduction device causes a higher temperature in the device. Personal portable optical recording media such as optical cards and the like, which have recently been developed, are not necessarily used in an office equipped with air-conditioning equipment. It is thus necessary to consider the situation where an optical recording medium is allowed to stand in a high-temperature environment for a long time. There is, therefore a need for an optical recording medium exhibiting greater stability in a high-temperature environment.